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Hydrogen is a high-quality energy carrier, similar to electricity, which can be used with high efficiency and near-zero emissions at the point of use. The most promising means of producing hydrogen using a renewable energy source and potentially reducing the generation of greenhouse gases production is through solar-hydrogen photoelectrochemical (PEC) water decomposition. In order to utilize the solar irradiation, small band gap material is essential. In this dissertation, I focus on the earth abundant, non-toxic and direct transit copper oxide (CuO) with band gap around 1.3-1.8 eV. In a PEC cell, the photo-excited charge carriers need to be separated as soon as they form in order to have a high photocurrent density. Thus, four approaches are studied: (1) decrease particle size to decrease the electron-hole recombination in the particles, (2) increase surface area to increase the active sites and decrease the distance for electrons travel to the surface to react with water (3) increase conductivity to decrease the resistance of the electrode, and (4) shorten charge carrier transport distance to decrease the chance of recombination of charge carriers.
In the first part of this study, I describe the aerosol route, flame spray pyrolysis, for making CuO nanoparticles. By controlling the precursor concentration and flame conditions, the particle size can be tuned. Also, the simulation results of particle growth, based on collision/sintering theory with sintering by solid state diffusion, are in good agreement with the experimental results. Furthermore, the flame spray pyrolysis made CuO nanoparticles were spin coated on conducting ITO glass substrates for the PEC study. Here, the relatively uniform CuO nanoparticles showed much better photocurrent density compared to the commercial CuO nanoparticles with a broad size distribution. This demonstrates the importance of the size of material for PEC application.
The second approach I introduced is to increase surface area to increase the active sites. Instead of changing the CuO suspension concentration to make films with different porosity, I present a new route for forming porous structures by spin coating the powder including CuO and its intermediate product, Cu2(NO3)(OH)3. During the post annealing process, the intermediate product transforms into CuO and leaves voids in the film, thus producing a porous film and increasing the active surface area for the water splitting reaction.
In the third approach, lithium was incorporated as a dopant to increase conductivity and decrease the resistance of the electrode. With the lithium added, the conductivity increased by two orders of magnitude and thus highly decreased the film resistance and increased the photocurrent density.
The final part of this dissertation focuses on three dimensional current collectors, used to decrease the charge carrier transport distance and thus decrease the chance of recombination. Here, the genetically modified tobacco mosaic virus (TMV1cys) served as a template for the three dimensional structure, made by sputter deposition of CuO. By varying the virus concentration, the distance between the current collectors can be tuned to optimize the charge carrier transport distance, light reflection as well as the CuO thickness for efficient absorption of solar energy.